As a method of mounting electronic components on a board, one mode has been employed widely in which bumps provided at the electronic component are joined to joining terminals formed on the board, respectively. Au, which is excellent in conductivity and never subjected to the surface deterioration due to oxidization, is widely used as the material of the bumps. As a method of joining bumps made of Au to joining terminals, there has been known a method using anisotropic conductive material which is formed by mixing conductive particles into thermosetting resin. In this method, the anisotropic conductive material is applied in advance on the board so as to cover the joining terminals prior to the mounting of the electronic components, and then the bumps are heated while being pressed against the joining terminals at the time of mounting the electronic components. Thus, the bumps and the joining terminals are electrically made conductive therebetween via the conductive particles and further the main bodies of the electronic components are joined with the board by the hardened anisotropic conductive material.
This method of using the anisotropic conductive material has an advantage that both the electrical conductivity and the joining of the main bodies of the electronic components to the board can be performed simultaneously. However, in order to secure good conductivity and realize a low coupling resistance, this method is required to keep the contacting state between the bump and the joining terminal uniform as to all of the bumps. To this end, the part mounting method using the anisotropic conductive material requires a mounting equipment with a high mounting accuracy. Thus, there has been required a method which can mount electronic components each provided with Au bumps easily and with a high quality.
Therefore, in order to satisfy such a requirement, various methods have been proposed in each of which the conductive particles contained in the anisotropic conductive material is replaced by solder particles (see patent documents 1 to 4, for example). In each example of these patent documents, a sheet-shaped anisotropic conductive material formed by mixing the solder particles in resin is used, and at the time of mounting electronic components, the solder resin disposed between bumps and joining terminals are molten thereby to secure good conductivity.    [Patent Document 1] JP-A-8-186156    [Patent Document 2] JP-A-10-112473    [Patent Document 3] JP-A-11-4064    [Patent Document 4] JP-A-11-176879
However, in the method shown in each of the aforesaid examples, when Su solder having been widely used in recent years as the solder material constituting the solder particles is employed, there arises a problem that the intensity is degraded due to Kirkendall voids as explained below. That is, in an area near the joining boundary where the bump made of Au and the joining terminal are joined via the solder, the diffusion of Sn into Au proceeds with the lapse of time. In this case, in the solder joining using the anisotropic conductive material, since an amount of the solder particles contained in the anisotropic conductive material is smaller as compared with that used in the usual solder joining, the difference of density of Sn likely becomes large between the solder joining portion and the bump. As a result, the diffusion of Sn from the solder joining portion into the bump is promoted. Accordingly, fine voids occurs at portions of the solder joining portion where Sn diffuses into Au, which causes the large degradation of the joining intensity.